Characteristics and Impact Factors of Acid Rain in Beijing

The annual and seasonal characteristics of acid rain in Beijing during 2003-2008 and their relationships with the meteorological conditions and pollutant concentrations are investigated using data observed at Beijing Weather Observatory, Changping and Shangdianzi acid rain observation stations. The average pH of precipitation is lower than 5.6 from 2003 to 2008, and the variation in the annual average precipitation acidity displays a decreasing trend over the past 6 years. After 2007, the acid rain pollution becomes more serious in Beijing area, when the average pH of precipitation is lower than 4.5 at all the three observation stations. The annual average pH values of precipitation at Beijing Weather Observatory and Changping are higher than that at Shangdianzi Station in the past 6 years except for 2008. It shows that the acidification degree in background area is heavier than that in urban area. Seasonal and monthly variations in precipitation acidity, conductivity and the frequency of acid rain are observed in Beijing area. The average pH and K values of acid rain in summer and autumn are lower than those in spring and winter. The frequency of acid rain with pH < 4.5 and 4.5 < pH < 5.0 is highest in autumn and summer, respectively.The monthly variation of acid deposition show that the heaviest acid rain pollution occurs in summertime, espe-cially in August.The decrease tendency in the pH and K value and the increase tendency in the frequency of acid rain with pH < 4.5 are found with the increase of precipitation intensity, respectively.The higher pH and K value at Beijing Weather Observatory and Changping Station indicate that the process of below-cloud scavenging is more important in urban area, and alkaline coarse particles in the atmosphere plays an important role in neutralization.The wind direction at 1500 ml evel has significant influence on the precipi-tation acidity and conductivity value over Beijing area.The lowest pH is observed with south wind, and higher K value and heavier acid deposition are recorded with the southwest and south wind.The transport of pollutants by the southerly flow enhances the acidity of precipitation.The acid rain is also influenced by the temperature inversion.The frequency of acid rain increases when continuous inversion occurs (temper-ature inversion occurs at rainy day and the day before that).Temperature inversion occurs most frequently during autumn that leads to the increase of acid rain pollution in this season.Furthermore, pollutants in the atmosphere have important effect on the acidity of precipitation.There is a significant negative correla-tion between the concentration of atmospheric pollutants, suchas SO₂, NO₂ and PM_2.5, and the pH of rain, suggesting that the variation of pollutant concentration in lower atmosphere layer has significant in-fluence on the acid rain.SO₂ and NO₂ are the important precursors of the acidic compositions and the major contributors of acidity in rainwater.The neutralization effect of fine particles (PM_2.5) is very weak.On the contrary, the acid compositions of PM_2.5 are partly contributed to the acidity of precipitation when they are scavenged into the rainwater.